Several critical processes occur during the four stages of mitotic cell division, which are prophase, metaphase, anaphase and telophase, including, without limitation, separation of the centrosomes and preparation of the cell to form the mitotic spindle; alignment of the chromosomes on the spindle in metaphase; and sister chromatid separation in anaphase. Specifically, during prophase the duplicated centrosomes migrate along the periphery of the nucleus towards opposite poles of the cell. During prophase the cell may also prepare for chromosome condensation and for other events that occur in metaphase. A critical and irreversible event during the transition from metaphase to anaphase is the irreversible segregation of sister chromatids between daughter cells.
The fidelity of mitosis is monitored by checkpoint genes. For example, a multitude of evolutionarily conserved checkpoint genes monitor the metaphase to anaphase transition. Several of these checkpoint genes have been identified, initially in yeast, and later in higher eukaryotes, that prevent the onset of anaphase until the mitotic spindle is properly assembled [Elledge, 1998, Science, 279:999–1000; Amon, 1999, Curr. Opin. Genetics Dev., 9:69–75]. The presence of these checkpoint genes, coupled with the predisposition towards aneuploidy when these checkpoint genes are inactivated, provide evidence that this transition is clearly an important milestone for mitosis. Although most of the research on mitotic checkpoints has focused on the spindle checkpoint, which monitors the transition from metaphase to anaphase, given the complexity of the mitotic process, the existence of additional checkpoints that monitor other phases of mitosis is likely. A checkpoint monitoring the anaphase-to-telophase transition has been described [Muhua, L. et al, 1998 Nature 393: 487–491].
Errors during mitosis can result in unequal chromosome segregation and are probably responsible for the aneuploid phenotype of cancer cells. Agents that target microtubules induce mitotic stress and thus cause such errors [McIntosh, J. R. & Koonce, M. P., 1989 Science, 246:622–628; Jordan, M. A. & Wilson, L., 1998 Curr. Opin. Cell Biol., 10: 123–130]. Many human cancers are sensitive to mitotic stress. This sensitivity is being exploited for therapy and implies that tumor cells have mitotic checkpoint defects [Lengauer et al., 1998, Nature, 396:643–649; Hartwell, L. H. & Kastan, M. B., 1994 Science, 266:1821–1828; Lengauer, C. et al, 1997 Nature 386:623–627; Lengauer, C. et al, 1998 Nature 396:643–649; Elledge, S. J. 1998 Science 279: 999–1000; Amon, A. 1999 Curr. Opin. Genet. Dev. 9: 69–75; and Li, Y. & Benezra, R., 1996 Science, 274: 246–248]. However, the known mitotic checkpoint genes, which prevent entry into anaphase when the chromosomes are not properly aligned on the mitotic spindle, are rarely inactivated in human cancer [Yamaguchi, K. et al, 1999 Cancer Lett. 139:183–187; Jin, D. Y. et al, 1998 Cell 93:81–91; Zou, H. et al, 1999 Science 285, 418–422]. For example, many of the mitotic spindle checkpoint genes have been examined for mutations in human cancer, but so far only infrequent bub 1 mutations have been detected [Cahill et al, 1998, Nature, 392:300–303; Cahill et al., 1999, Genomics, 58:181–187]. Thus, the molecular basis of cancer aneuploidy remains elusive, except for the small number of cases with bub 1 mutations.
Thus, there remains a need in the art for the identification of additional methods and compositions useful in the diagnosis of cancer, particularly the identification of additional genes that monitor and control mitosis, as well as methods and compositions that permit the screening of drugs useful for treatment of cancer. The present invention satisfies this need.